An equation is a mathematical statement that has two expressions separated by an
equal sign. The expression on the left side of the equal sign has the same value as
the expression on the right side.

One or both of the expressions may contain variables. Solving an equation means
manipulating the expressions and finding the value of the variables.

For example solve the equation: 7x = 21
To keep both sides of an equation equal, we must do exactly the same thing to each side of
the equation. If we multiply (or divide) one side by a quantity, we must
multiply (or divide) the other side by that same quantity.
In order to solve this equation we would divide both sides by 7. The equation
would become 7x/7 = 21/7. When simplified, this would become x = 21/7 or x = 3.
It is possible to substitute the value of x back into the original equation
7*3=21.